This invention relates to a sheet conveyor for use in copying machines and the like and adapted for conveying a sheet such as a transcription sheet while the leading edge of the sheet is held by means of a gripper.
Although a conveyor of the above type, for example, a chain delivery type conveyor for use in copying machines will be discussed in the following description with reference to FIG. 1, it should be understood that the present invention is also applicable to any other sheet conveyors.
In FIG. 1, there is illustrated a basic configuration of the chain delivery type sheet conveyor which includes a photosensitive drum 1 rotating in the arrow direction. Around the photosensitive drum 1 there are disposed a corona charger 2 for charging a surface of the photosensitive drum 1 with a voltage, a light exposure optical system 4 including a mirror, a lens, etc, for guiding and focusing onto the surface of the drum 1 an image of an original (not shown) mounted on an original platform 3. In addition, a developer 5 is provided for making visible an electrostatic latent image formed via the optical system, and a sheet conveyor 8, which is directed to the present invention, is disposed in closest proximity to the photosensitive drum 1 within a transcription station 6 for bringing the sheet into contact with the visible image and further conveying the thus transcribed sheet into a fixing station 7 and delivering the same outside. Further a corona charger 9 is disposed within the transcription station 6 for ensuring the efficiency of the transcription operation. Still further a corona charger 10 is provided for removing the residual charge from the photosensitive drum 1 after the transcription operation and a cleaner 11 for removing residual developing agents from the photoconductive surface of the photosensitive drum 1, both of which are also located around the photosensitive drum 1.
The sheet conveyor 8, as indicated in FIG. 2, includes a pair of chains 12 and two pairs of sprockets 13, 14 one pair of which is designed to drive the chains 12. The conveyor 8 further includes a sheet gripper 15 clinging to the chains 12 in a direction normal to the chains. The sheet gripper 15 is adapted to rotate in the direction of the arrow in FIG. 1 and stop rotating just before the transcription station 6.
On the right side of FIG. 1 there is provided a sheet feed assembly including a sheet feed roller 16 rotating in the direction of the arrow and a sheet cassette 17 containing a stack of sheets. Guide rollers 18 and auxiliary rollers 19 are located in a sheet passage between the sheet feed roller 16 and the sheet gripper in a ready position. As soon as the working surface of the photosensitive drum is charged with a given voltage and the leading edge of an image section of the photosensitive drum 1 reaches an optical path of the exposure optical system 4, the original platform 3 starts moving so that the image of the original is projected onto the working surface of the photosensitive drum 1 via the optical system 4. The resulting latent image is developed into a corresponding toner image through the developer 5.
Concurrently with or shortly after the actuation of a pushbutton the sheet feed roller 16 makes a full revolution to pick up only a single sheet 20 from the sheet cassette 17 and move it forward towards the sheet gripper 15 in the ready position. At the same time the sheet conveyor 18 starts rotating in order to bring into synchronism the time when the leading edge of the toner image section reaches the transcription station 6 with the time where the sheet 20 is conveyed into the transcription station 6.
Thereafter, a toner image attached to the working surface of the drum is transferred onto the sheet 20 of which the tip is held by the sheet gripper 15, by the action of the transcription corona charger 9. A residual potential is removed from the photosensitive drum 1 by the corona charger 10 after the completion of the transcription operation. Residual toner is also removed from the working surface of the drum through the cleaner 11. The sheet 20 carrying the toner image transferred thereon is conveyed into the fixing station 7. After the toner image has been heated and fixed, the sheet gripper 15 reaches the sprockets 13 to release the sheet 20 therefrom. Until the leading edge of the sheet 20 travels towards the sheet drain rollers 21, the sheet drain auxiliary rollers 22 are at work to advance the sheet. Then, the sheet 20 is discharged from the copying machine through the sheet drain rollers 21.
FIG. 3 depicts in detail how the sheet gripper 15 holds the sheet 20 within the copying machine of the chain delivery type. In FIG. 3, an attachment 23 is provided to support the gripper 15 which comprises a base plate 15a affixed to the attachment 23 and a sheet pressure plate 15b pivoted rotatable on the base plate 15a. The base plate 15a itself is concave and secured via a screw 24 on the attachment 23 which in turn is affixed to the pair of the chains. A sheet pressure bearing 25 is provided on the sheet conveying direction side of the base plate 15a and carries a rotatable sheet pressure shaft 26. The sheet pressure plate 15b is secured via a screw 27 on the sheet pressure shaft 26. A cam follower 28 is fixed together with the sheet pressure plate 15b to the sheet shaft 26 via a screw. A roller 29 is provided below the follower 28. The base plate 15a also has an inwardly folded sheet receiving arm 30 and a downwardly folded sheet tip restricting arm 31 on its sheet conveying direction side. A cam 32 is provided in a position to contact the roller 29 extending from the cam follower 28 on the sheet pressure plate 15b.
As illustrated in FIG. 3(a) the chains are in the stopped position and the sheet gripper 15 is in the ready condition. Under the circumstance the roller 29 extending from the cam follower 28 rides on the cam 32 so that the sheet pressure plate 15b is rotated about the sheet pressure shaft and placed in an open position. When this occurs, the sheet 20 strikes on the sheet tip restricting arm 31 and stops at that position.
Upon driving the chains the roller 29 passes over the cam 32 and the sheet pressure plate 15b is urged against the base plate 15a under the influence of a spring 33 intervened therebetween to sandwich the sheet 20 therebetween. When passing the transcription station the sheet 20 receives the toner image transcribed thereon and the toner image is then fixed at the fixing station. In discharging the sheet the cam 32 is placed in a sheet release position and the sheet pressure plate 15b is rotated as shown in FIG. 3(a).
As noted earlier, the sheet gripper structure of the conventional conveyor including the cams, the cam follower, the rotation shaft, the bearing, etc. is rather sophisticated and requires that the sheet be securely sandwiched by means of the spring. The cams are necessary in the sheet hold position and in the sheet release position with position adjustment therefor. The spring mounted on the sheet gripper becomes fatigued gradually due to heat each time it passes over the fixing station. In an extreme case the sheet gripper structure fails to catch the sheet and causes a sheet jam which may produce fire or smoke.